In the past, the only alternative for a center roll in a press section of a paper machine was a rock roll, which was usually made of granite. The popularity of granite was based on its surface properties which enable controlled separation of the paper web from the rock face and, moreover, granite tolerates the abrading effect of a doctor which is used in conjunction with a center roll very well. However, granite has certain drawbacks. Specifically, since it is a natural material, its properties show variation, and internal flaws in granite and its tendency of cracking form a series obstacle for its use in some applications. For this reason, today, as the center roll of the press section of a paper machine, a coated roll is used very frequently, in particular a roll coated with an oxide coating.
In the art of the production of paper, coated rolls are also used in a great variety of other applications, for example as press rolls, suction rolls, and as rolls of calenders and supercalenders.
The coating can be added in a number of different manufacturing processes onto the various rolls employed in the production of paper, and specifically onto a roll frame of the roll. One important process for adding the coating onto a roll is thermal spraying. However, in this type of coating process, the problem occurred that a certain degree of porosity has tended to remain in the roll coatings, i.e., the thermal spraying results in pores in the coating. This tendency occurs in particular in coatings prepared from oxide ceramic by thermal spraying, because oxide materials are poorly meltable. Oxide coatings are, however, highly suitable roll coatings for applications in which requirements are imposed concerning separation of the paper web, resistance to wear, resistance to corrosion, doctoring, and keeping clean. Such applications are, for example, the center roll in a press section of a paper machine and the rolls in calenders.
It has been one of the more prominent problems in thermally sprayed roll faces that the pores in the roll face are readily filled with contaminants existing and forming in connection with the manufacture of paper, for example pitch, or pastes contained in recycled pulps. When the pores in the roll coating are contaminated, the properties of the coating can be changed and, moreover, there is the risk that the contaminants/moisture break the boundary faces between the pores, in which case the coating is also worn by cracking and splitting.
It is one prior art application for sealing the pores in the coatings that so-called sealing agents are made to penetrate into the porous face. One prior art application of such a sealing agent consists of thermosetting plastics which are composed of two or more components and which have a low viscosity when not solidified. In roll faces sealed in this manner, problems have, however, arisen from the fact that the suitability for doctoring is deteriorated because the plastic face tends to become smooth. Moreover, the smoothing may cause problems of runnability, because the properties of separation of paper are also changed.